This invention relates to a method and apparatus for removing sulfur from hydrocarbon-containing fluid streams using fluidizable and circulatable solid particles. In another aspect, the invention concerns in a hydrocarbon desulfurization unit having an improved design that reduces capital expense and operating expense while providing for enhanced sulfur removal and particle circulation.
Hydrocarbon-containing fluids such as gasoline and diesel fuels typically contain a quantity of sulfur. High levels of sulfurs in such automotive fuels are undesirable because oxides of sulfur present in automotive exhaust may irreversibly poison noble metal catalysts employed in automobile catalytic converters. Emissions from such poisoned catalytic converters may contain high levels of non-combusted hydrocarbons, oxides of nitrogen, and/or carbon monoxide, which, when catalyzed by sunlight, form ground level ozone, more commonly referred to as smog.
Much of the sulfur present in the final blend of most gasolines originates from a gasoline blending component commonly known as “cracked-gasoline.” Thus, reduction of sulfur levels in cracked-gasoline will inherently serve to reduce sulfur levels in most gasolines, such as, automobile gasolines, racing gasolines, aviation gasolines, boat gasolines, and the like. Many conventional processes exist for removing sulfur from cracked-gasoline. However, most conventional sulfur removal processes, such as hydrodesulfurization, tend to saturate olefins and aromatics in the cracked-gasoline and thereby reduce its octane number (both research and motor octane number). Thus, there is a need for a process wherein desulfurization of cracked-gasoline is achieved while the octane number is maintained.
In addition to the need for removing sulfur from cracked-gasoline, there is also a need to reduce the sulfur content in diesel fuel. In removing sulfur from diesel fuel by conventional hydrodesulfurization, the cetane is improved but there is a large cost in hydrogen consumption. Such hydrogen is consumed by both hydrodesulfurization and aromatic hydrogenation reactions. Thus, there is a need for a process wherein desulfurization of diesel fuel is achieved without significant consumption of hydrogen so as to provide a more economical desulfurization process.
Recently, improved desulfurization techniques employing regenerable solid sorbents have been developed to meet the above-mentioned needs. Such regenerable sorbents are typically formed with a metal oxide component (e.g., ZnO) and a promoter metal component (e.g., Ni). When contacted with a sulfur-containing hydrocarbon fluid (e.g., cracked-gasoline or diesel fuel), the promoter metal and metal oxide components of the regenerable sorbent cooperate to remove sulfur from the hydrocarbon and store the removed sulfur on/in the sorbent via the conversion of the metal oxide component (e.g., ZnO) to a metal sulfide (e.g., ZnS). The resulting “sulfur-loaded” sorbent can then be regenerated by contacting the sulfur-loaded sorbent with an oxygen-containing regeneration stream. During regeneration, the metal sulfide (e.g, ZnS) in the sulfur-loaded sorbent is returned to its original metal oxide form (e.g., ZnO) via reaction with the oxygen-containing regeneration stream. Further, during regeneration the promoter metal is oxidized to form an oxidized promoter metal component (e.g., NiO). After regeneration, the oxidized sorbent can then be reduced by contacting the oxidized sorbent with a hydrogen-containing reducing stream. During reduction, the oxidized promoter metal component is reduced to thereby return the sorbent to an optimum sulfur-removing state having a metal oxide component (e.g., ZnO) and a reduced-valence promoter component (e.g., Ni). After reduction, the reduced sorbent can once again be contacted with the sulfur-containing hydrocarbon fluid to remove sulfur therefrom.
Traditionally, solid sorbent compositions used in hydrocarbon desulfurization processes have been agglomerates utilized in fixed bed applications. However, because fluidized bed reactors provide a number of advantages over fixed bed reactors, it is desirable to process hydrocarbon-containing fluids in fluidized bed reactors. One significant advantage of using fluidized bed reactors in desulfurization systems employing regenerable solid sorbents is the ability to continuously regenerate the solid sorbent particles after they have become “loaded” with sulfur. Such regeneration can be performed by continuously circulating the solid sorbent particles from a reactor vessel, to a regenerator vessel, to a reducer vessel, and then back to the reactor. Thus, employing a sorbent composition that is both fluidizable and circulatable allows for substantially continuous removal of sulfur from a hydrocarbon-containing fluid stream and substantially continuous sorbent regeneration.
When designing a desulfurization unit employing a fluidized bed reactor, a fluidized bed regenerator, and a fluidized bed reducer which provide for continuous sulfur removal via fluidizable and circulatable solid sorbent particles, many design parameters must be considered. One of the main considerations in designing any desulfurization unit is the initial capital cost of the unit. The number of vessels, valves, conduits, and other equipment in the unit contributes significantly to the capital cost of a desulfurization unit. Further, the elevation of the individual vessels in a desulfurization unit can contribute significantly to the capital cost of the desulfurization unit because the support structure for supporting large vessels high above the ground can add considerably to the construction and maintenance costs of the unit.
Another important consideration in designing a desulfurization unit is operating cost. Complex particle transport systems (e.g., pneumatic conveyors) can increase operating costs due to frequent maintenance and/or breakdowns. In desulfurization units employing fluidizable and circulatable solid particles to remove sulfur from a hydrocarbon-containing fluid, particle attrition can cause also increased operating cost. Generally, attrition of solid particles is increased when solid particles are transported at high velocity. Thus, desulfurization units that employ dilute phase transport of the solid particles through and between vessels can cause significant attrition of the particles. When the solid particles employed in the desulfurization unit experience high levels of attrition, the solid particles must be replaced at frequent intervals, thereby increasing operating cost and downtime of the unit.